WASHINGTON, Dec 27 (Reuters) - U.S. Environmental ProtectionAgency chief Lisa Jackson, who spearheaded the Obamaadministration's crackdown on carbon emissions, said on Thursdayshe will step down after almost four years of battles withRepublican lawmakers and industry over proposed regulations.

Under her leadership, the agency declared for the first timethat carbon dioxide was a danger to human health and could beregulated under the Clean Air Act, leading the EPA to develop anew regulatory regime to limit carbon emissions.

Industry groups and Republican lawmakers opposed Jackson'sefforts to fight climate change, hauling her in for numeroushearings in Congress, and she faced some pushback from withinthe administration too.

She won praise from many environmental groups, while otherscomplained her EPA was too timid. It was unclear what directionthe administration will take on climate change during PresidentBarack Obama's second term.

Obama thanked Jackson for her service, praising her work onmercury pollution limits, fighting climate change and helpingset new fuel economy standards for vehicles.

"Under her leadership, the EPA has taken sensible andimportant steps to protect the air we breathe and the water wedrink," Obama said in a statement.

Jackson, the first black administrator of the 17,000-strongEPA, said in a statement she was "confident the (EPA) ship issailing in the right direction."

Jackson, 50, is expected to leave her cabinet position afterObama's State of the Union address in early 2013. Leading thelist of potential replacements are Bob Perciasepe, deputy EPAadministrator, who will take over the agency on an interimbasis; and Kathleen McGinty, a former head of Pennsylvania'sDepartment of Environmental Protection and a protege of formerU.S. Vice President Al Gore.

Also said to be in the mix are Gina McCarthy, the EPA'sassistant administrator for the Office of Air and Radiation; andMary Nichols, chair of the California Air Resources Board.

Jackson's departure was not a surprise. Analysts had notexpected her to stay for Obama's second term.

The administration is expected to face a tough fight to getany potential nominee confirmed by the Senate -- especially anycandidate seen as being in the mold of Jackson.

"Secretary Jackson played the environmental 'bad cop' toPresident Obama's more moderate 'good cop,' but the result oftheir tag-team effort has been a huge expansion of the EPA'spower. That's the exact opposite of what is needed," said S. T.Karnick, research director at the Heartland Institute, a Chicagogroup that is skeptical of man-made climate change.

Jackson is the first major energy policy official to stepaside since Obama's re-election last month. Some have speculatedthat Energy Secretary Steven Chu, a Nobel prize-winningphysicist who has also clashed with industry, will also depart,as may Interior Secretary Ken Salazar.

BRUISING ENCOUNTERS

Republican lawmakers accused Jackson's EPA of massivegovernment overreach that choked economic growth, and passednumerous bills aimed at undoing the regulations. Obama did notsign their bills into law, but the White House did begin to pullback or delay rules in the face of the relentless onslaught.

Some speculated Jackson would step down in 2011, when Obamadecided to delay rules to restrict emissions of smog-formingchemicals from power plants.

"From an energy and consumer perspective, it has to be saidthat the Jackson EPA presided over some of the most expensiveand controversial rules in agency history," said Scott Segal,director of the Electric Reliability Coordinating Council, whichlobbied against many of the EPA's proposed regulations.

States and governors fought Jackson's rules in the courts,scoring a win in August when a U.S. appeals court overturned theEPA's Cross-State Air Pollution Rule, aimed at reducing harmfulemissions from coal-burning power plants.

On Thursday, many environmentalists and public healthadvocates hailed Jackson, saying she leaves a legacy of cleanerair.

"Administrator Jackson has been one of the most effectiveleaders in the history of the Environmental Protection Agency,"Larry Schweiger, president of the National Wildlife Federation.

Jackson is a chemical engineer by training, and reports inrecent weeks suggested she might be under consideration for thepost of president of Princeton University. She is also aone-time chief of staff of New Jersey Governor John Corzine, andother media reports say she may be mulling a run for governor ofthat state.

Despite contentious dealings with Congress, Jacksonmaintained a cordial relationship with one of her biggestcritics, Senator Jim Inhofe. She even kept a photo of theOklahoma Republican and his grandchildren in her office.